1. Field of the Invention
The present invention relates to a measurement apparatus and a method of manufacturing an optical system.
2. Description of the Related Art
An interferometer is used to measure the transmission wavefront or reflection wavefront (optical characteristic) of an optical system. Measurement of the wavefront using an interferometer is important especially for the projection optical system of an exposure apparatus for manufacturing a semiconductor device using photolithography.
To measure and evaluate the surface shape error of an optical element and the transmission wavefront of an optical system at high precision using an interferometer, it is important to separate a wavefront error (so-called system error) unique to (the optical system of) the interferometer and the shape error of a member to be measured (surface to be measured).
For example, a wavefront average method is proposed as a method of separating the system error and the shape error of a member to be measured. According to this method, the wavefront is measured by a plurality of number of times while displacing at random the measurement region of the member to be measured or a Newton gauge surface with respect to the optical axis of the interferometer. There is also proposed a method (to be also referred to as a “rotation shift method”). In the rotation shift method, the wavefront is measured in a plurality of arrangements obtained by rotating and shifting the member to be measured or the Newton gauge surface with respect to the optical axis of the interferometer. Then, the rotation asymmetrical component and rotation symmetrical component are calculated separately. These techniques are disclosed in Japanese Patent Laid-Open No. 5-223537.
In wavefront measurement using an interferometer, if a change of the system error over time is negligibly small with respect to the required measurement precision, the system error is calibrated periodically at a relatively long period. However, if a change of the system error over time cannot be ignored with respect to the required measurement precision or a Fizeau lens which forms part of the optical system of the interferometer needs to be exchanged, the system error has to be calibrated more frequently. In some cases, the system error needs to be calibrated every time the target member is measured.
In system error calibration, the member to be measured or the Newton gauge surface needs to be measured in a plurality of arrangements with respect to the optical axis of the interferometer by the foregoing wavefront average method, rotation shift method, or the like. Hence, frequent calibration of the system error prolongs the time taken to measure one target member, and the member cannot be measured efficiently.